1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) and to a method for manufacturing such LCDs, and in particular to a peripheral driving circuit integrated LCD in which a thin film transistor (TFT) is used as a switching element in a display area and a driving circuit is formed around the display area, and a method for manufacturing the same. The manufacturing method of this invention prevents dielectric breakdown of an element due to static electricity generated in the manufacturing process.
2. Description of the Related Art
LCDs are commonly employed in office automation and audio visual apparatuses because of their advantageously small size, thin shape, and low power consumption. In particular, active matrix LCDs employing a TFT for a switching element to control writing of pixel information into pixels, are used for displays of various television sets or personal computers as they can precisely display motion pictures on a large screen.
A TFT is a field effect transistor (FET) made by forming metallic and semiconductor layers of a predetermined shape on a insulating substrate. In an active matrix LCD, the TFT is connected to the pixel electrodes for driving liquid crystal. Note that a common electrode, a pixel electrode, and liquid crystal sandwiched by these, together constitute a capacitor which corresponds to one pixel.
In recent years, an LCD has been developed which employs polysilicon (p-si) for the semiconductor layer, instead of amorphous silicon which was mainly used. Laser light is used to anneal, form, and grow p-Si crystal. In general, p-Si is superior in carrier movability to a-Si, and achieves TFT size reduction which enables formation of a highly precise, fine LCD with a higher opening rate. Moreover, when a gate self-align structure enables formation of a fine structure, and reduced parasitic capacitance enables high-speed processing, it is possible to form a high speed driving circuit through employment of an electric complementary structure which uses an n-ch TFT and a p-Ch TFT, i.e., CMOS. This further allows formation of a driving circuit around a pixel area on the same substrate, so that manufacturing costs and the size of an LCD module can be reduced.
Referring to FIG. 11, which is a plan view of a mother substrate 1 of the aforementioned driver-integrated LCD, the mother substrate 1 includes four active matrix substrates 2 which constitute electrode substrates of LCDs on one. On each active matrix substrate 2, respective areas are reserved for formation of a display area at the center, gate driver 40 on the left and right sides thereof, a drain driver thereabove, a precharge driver 6 therebelow, an input terminal area 7 along the lower edge of the substrate 2. The input terminal area 7 is connected to a flexible print connector (FPC), which is mounted with an integrated circuit for generating a control signal to be supplied via the FPC to the input terminal area 7.
From the input terminal area 7, a vertical clock pulse feeding wire 41 and a vertical start pulse feeding wire 42 extend to the gate driver 4; a horizontal clock pulse feeding wire 51, a horizontal start pulse feeding wire 52, and a video data feeding wire 53 extend to the drain driver 5; and a horizontal clock pulse feeding wire 61 and a horizontal start pulse feeding wire 62 extend to the precharge driver 6.
After an opposing glass substrate is attached to the motor substrate 1, the substrate 1 is cut along the break line 8 into four sheets of active matrix panels. Note that the opposing glass substrate has common electrodes formed thereon correspond to the substantial area of each active matrix substrate.
Referring to FIG. 12 which is an enlarged plan view of an active matrix substrate 2, a display area 3 is formed such that horizontally extending gate lines 31 intersect vertically extending drain lines 32, and a switch element 33 is provided at each crossing, connected to the pixel electrode for driving crystal liquid.
A gate driver 4 mainly comprises a shift register for supplying a scanning signal voltage to the gate lines 31 in response to a vertical clock pulse. A drain driver 5 mainly comprises a shift register and a sampling gate for supplying a display signal voltage to the drain lines 32 in response to a horizontal clock pulse.
A precharge driver 6, comprising mainly a shift register, is provided, when necessary, to supply the display signal voltage to the drain lines 32 earlier than the drain driver 5 to eliminate residual voltage in the drain lines 32 since previous scanning periods.
In the input terminal area 7, input terminals 71 are arranged respectively connected to the wires 41, 42, 51, 52, 53, 61, 62.
Each switch element 33 comprises, for example, a TFT, and all switch elements 33 in the same row are collectively turned on by a scanning signal voltage, in synchronism with which the display signal voltage is applied from the drain lines 32 to each pixel electrode 34. By using the applied voltage as display information, permeability of liquid crystal in each pixel is controlled so as to display an image using bright and dark pixels.
A driver for the aforementioned driver-integrated LCD is made by forming a p-Si (polysilicon) TFT on a substrate. That is, a CMOS is formed using a pair of TFTs each having the same structure as that of a TFT used for a switch element 33 in the display area so that a number of inverter circuits are formed on a single substrate, forming respective drivers 4, 5, 6.
Referring to FIG. 13, which is a cross sectional view of major elements of the aforementioned active matrix substrate 2, from left to right in the drawing are shown a TFT area, a wire 41, 42, 52, 52, 53, 61, 61 area, and an input terminal 71. On a glass substrate 100, a gate electrode 101 and an input terminal pedestal 121 are formed as a first conductive layer made of Cr or the like. Above them, a gate insulating film 102, a p-Si film 103, an injection stopper 104, an interlayer insulating film 105, a source electrode 106, a drain electrode 107, a wire 116, an input terminal 126, a flattening insulating film 108, a pixel electrode 109, and an input terminal contact film 129 are formed. The source electrode 106, the drain electrode 107, the wire 116, and the input terminal 126 are made of Al or the like to serve as a second conductive layer; the pixel electrode 109 and the input terminal contact film 129 are made of indium tin oxide (ITO).
As can be seen from this drawing, the input terminal 71 has a three-layer structure including an input terminal pedestal 121, an input terminal 126, and an input terminal contact film 129. The input terminal 126, integrated with the wire 116, is made of a highly conductive Al or the like, which, however, is inferior in property of attaching to the substrate 100. Therefore, an input terminal pedestal 121 made of Cr, which adheres well to both Al and glass, is provided as a base of the input terminal 126 to ensure rigid adherence between the input terminal 126 and the substrate 100.
Because anisotropy conductive resin used as an adhesive member with an FPC is not easily used with the input terminal 126, an input terminal contact film 129 made of ITO is intervened so as to ensure better adherence with the FTC.
First, a gate line 31, a gate electrode 101, and an input terminal pedestal 121 may be formed. That is, a gate electrode 101 for a switching element and a gate line 31 integrated with the gate electrode 101 are formed in the display area 3; a gate electrode 101 for a CMOS TFT and lower wires for wire bonding are formed in the driver areas 4, 5, 6; and a pedestal 121 for an input terminal 71 is formed in the input terminal area 7. A source electrode 106, a drain electrode 107, drain lines 32, and wires 41, 42, 51, 52, 53 are not yet formed.
As can be seen from the structure shown in FIG. 3, in manufacturing an active matrix substrate, a lower electrode wire layer including a gate electrode 101 and an input terminal pedestal 121 are formed at the first stage, followed by many stages at which a p-Si film 103 and various insulating films 102, 104, 106 are formed and etched and further by subsequent stages at which an upper electrode wire layer including a source electrode 106, a drain electrode 107, and a wire 116, are formed. Through these stages, static electricity may be caused by friction with the mother substrate 1, particularly near the edges of the substrate. Especially, if an island-shaped input terminal pedestal 121 is charged, charged electricity is discharged toward the surrounding metal. Specifically, referring to FIG. 11, for example, TFT elements constituting a precharge driver 6 and a drain driver 5 of the adjacent active matrix substrate 2 are subject to the influence of the discharged electricity from the input terminal area 7 as they are positioned close to the input terminal area 7, especially at a stage with a gate electrode 101 formed. Discharged static electricity would deteriorate the element characteristics and cause dielectric breakdown, particularly at a stage where a p-Si film 103 has been formed.
Drain drivers 5 on the upper active matrix substrates 2 in FIG. 11 are also more likely affected by the static electricity as they are positioned near the edge of the mother substrate 1, i.e., close to a part touched by a man""s hand or a supporting section of a device.
The present invention has been conceived to with an aim of preventing deterioration of element characteristics due to static electricity generated in manufacturing.
According to the present invention, a conductive section is formed for discharging electricity, when forming a TFT so that the static electricity generated at the edges of a substrate is absorbed and shielded by the conductive section. With this arrangement, breakdown of the TFT due to static electricity is prevented.
Particularly, a conductive section formed near the edge of the substrate could effectively prevent the TFT from breakdown.
Further, the conductive section may preferably be formed as a part of a wire arranged on the substrate. Also, an input terminal constructed to serve as a conductive section may be used to effectively prevent a TFT from breakdown due to static electricity even though the lower layer of the input terminal is charged with static electricity.
Still further, a conductive section formed on the mother substrate may be used to prevent a TFT from breakdown due to static electricity generated in the mother substrate. Also, when an unnecessary conductive section is disposed of, a finished display apparatus is not affected by the conductive layer as it does not include the conductive layer.